Article Figures & Data

Figures

Brain functional network changes associated with differently effortful cognitive tasks (zero-, one-, and two-back working memory trials) in the β-band frequency interval (16–32 Hz). A, Thresholded synchronization matrices can be represented graphically as a network in anatomical space (F, front; L, left, R, right): each node represents an MEG sensor and there is a line or edge drawn between nodes if β-band oscillations are highly synchronized between sensors. The diameter of each node indicates its degree; nodes are colored to indicate their membership of topological modules. B, Node size indicates local efficiency or clustering coefficient, a measure of segregated network topology. C, Long-distance connections, at least as long as the 95th percentile of the distance distribution in the two-back network, are highlighted as black edges. D, Modularity is represented by color-coding the connections between nodes: intermodular connections are red and intramodular connections are gray. Right column, Statistical significance of task-related differences in local (nodal) network properties are shown for local efficiency (B), physical distance (C), and proportion of intramodular edges (D). In these panels, the size of each node corresponds to the log P value for a t test of the null hypothesis that the task-related difference in network metrics is zero. The most effortful two-back task is associated with less clustering, longer-distance synchronization, and a larger proportion of intermodular connections compared with the least effortful zero-back task.

Cognitive load effects on β-band network properties in two groups of fast- and slow-performing participants. Both fast- and slow-performing subgroups of participants demonstrate workspace configuration of networks at higher cognitive load, but this task-related effect on efficiency and physical distance is greater in the fast-performing group (high) than in the slow-performing group (low). The dotted lines indicate zero difference in network metrics between different levels of task difficulty. *p < 0.05 and **p < 0.01 for the null hypothesis that task-related change in network metrics is not different between fast- and slow-performing subgroups. Box sizes represent first and third quartile; whiskers show extent of distribution; black dot is the median and blue dots are outliers.

Dynamic changes in workspace configuration of β-band networks related to changing levels of cognitive effort in the course of a single (1800 ms) trial. Center, Period of visual stimulus presentation (gray bar) and the frequency distributions of latency of button press response in the zero-back (red line) and two-back (blue line) tasks. Note that most participants executed a motor response by the end of the first half of the trial (thick dashed line). Left, Zero-back (top) and two-back (bottom) networks measured at time ∼500 ms. During this response generation phase of the trials, both tasks elicited workspace configuration of brain networks. This is represented by low clustering (small diameter of most nodes) and topological proximity of nodes from anatomically different regions [node colors code anatomical location of sensors (top center)]. Right, Zero-back (top) and two-back (bottom) networks measured at time ∼1350 ms in the working memory phase of the task demonstrate different topologies: the two-back network maintained a workspace configuration whereas the zero-back network relaxed to a more locally clustered configuration. This is represented by the greater clustering (node diameter) of many nodes and greater topological proximity of nodes from the same anatomical regions in the zero-back network. Bottom row, Changes in key workspace parameters as a function of trial duration for zero-back (red lines) and two-back (blue lines) versions of the task: from left to right, global efficiency, local efficiency or clustering, physical distance, and proportion of intramodular edges. Red bar, Interquartile range of response latencies. L, Left; R, right; F, front.

Time-resolved changes in β- and γ-band network topology during zero- and two-back tasks. Networks oscillating at β- and γ-band frequencies demonstrate significant task-related differences in network metrics during the second half (working memory phase) of each trial but not during the first half (response generation phase). ***p < 0.001, **p < 0.01, and *p < 0.05 for the null hypothesis that metrics are not different in the zero-back (red box), one-back (green box), or surrogate (S; gray box) networks compared with the two-back networks (blue box). Box sizes represent first and third quartile; whiskers show extent of distribution; black dot is the median and blue dots are outliers.

Detailed ANOVA statistics are presented for two-way mixed effect models estimated with the connection density held constant across all conditions at 10% of maximum connection density. For comparison, the Pτ values are given for the same analyses repeated for networks with the synchronization threshold Cτ held constant across conditions, allowing some variation of connection density around 10%.